1. Field of the Invention
The present invention relates to an analog/digital-signal circuit including an analog-signal circuit and a digital-signal circuit.
2. Related Art
Such an analog/digital-signal circuit in the related art is, for example, a demodulating circuit for demodulating a digital signal obtained as a result of modulation through a digital modulating process such as phase-shift keying (which will be abbreviated to "PSK", hereinafter).
The PSK modulating method will now be briefly described. To transmit a digital signal, it is necessary to modulate a carrier wave using a pulse series of the digital signal. One method of methods for the modulation is the PSK modulation method. The PSK modulation method is a pulse modulation method, that is, a method for modulating the carrier wave using the pulse series, of a well-known PM (Phase Modulation) method, one of methods AM, FM and PM used for modulating a carrier wave with a signal including an analog signal. FIGS. 3A, 3B and 3C indicate wave shapes resulting from modulating the carrier wave using a pulse signal of a pulse series (0, 1, 1, 0, 1, 0, 0, 1), according to AM (Amplitude Modulation), FM (Frequency Modulation), and PM, being called respectively ASK (Amplitude Shift Keying), FSK (Frequency Shift Keying), and PSK for the pulse modulation. A received sine-wave PSK signal such as that shown in FIG. 3C is converted into a rectangular-wave through well-known pulse technology, and an input signal to the circuits shown in FIGS. 1 and 2 is produced.
FIGS. 4A and 4B show a 2PSK method and a 4PSK method, respectively. In the 2PSK method, as shown in FIG.4A, as a result of the modulation, a "0" of the pulse series corresponds to a phase of the modulated carrier wave rotated by .pi. radians, that is, 90.degree., from a phase of the modulated carrier wave corresponding to a "1" of the pulse series. In the 4PSK method, as shown in FIG.4B, four different phases, rotated by .pi./2radians, that is, 45.degree., from one another, of the modulated carrier wave are used to carry the pulse signal.
With reference to FIG. 1, the demodulating circuit will now be described. The demodulating circuit includes a first digital logic circuit 101, a low-pass filter 102, an analog-to-digital converter (which will be referred to as an A-D converter, hereinafter) 103, and a second digital logic circuit 104. The two digital logic circuits 101 and 104 are driven by a digital-system power source 110, and the low-pass filter 102 and the A-D converter 103 are driven by an analog-system power source 111. A reference top voltage is supplied by a voltage source 120 to the A-D converter 103 and a reference bottom voltage is supplied by a voltage source 121 to the A-D converter 103.
Operation of the above-described circuit will now be described. The first digital logic circuit 101 generates a pulse signal carrying phase comparison information which is obtained as a result of mixing An input signal and a reference signal through a digital mixer. The pulse signal carrying phase comparison information is such as that shown in FIGS. 6 and 7 described later. The digital mixer, simply consisting of an exclusive-OR (XOR) operation unit, obtains a phase difference between the input signal and the reference signal. The resulting pulse signal is converted into an analog signal through the low-pass filter 102 as a result of the low-pass filter 102 removing high-frequency components from the pulse signal. The analog signal is quantized through the AD converter 103 in a range between the reference top voltage and reference bottom voltage supplied by the voltage sources 120 and 121. The quantized data is appropriately processed by the second digital logic circuit so that data reproduction is performed from the input signal.
A principle of the data reproduction operation performed by the demodulating circuit shown in FIG. 1 will now be described with reference to FIGS. 5, 5-7. The digital mixer of the logic circuit 101 consists of the XOR operation unit (XOR) as mentioned above. As shown in FIG. 5, the modulated signal (MODSIG) and the reference signal (REFSIG) are input to the XOR (XORSGI), and the output of the XOR is filtered by the LPF 102 and A-D converted by the A-D converter 103. As shown in FIG. 6, if a phase difference .delta..o slashed. between the modulated signal and the reference signal is relatively small, each of pulse spans of the output of the XOR is short accordingly. The short pulse spans are integrated by the LPF 102, respectively, and thus the output of the LPF 102 has a small value. If the phase difference .delta..o slashed. between the modulated signal and the reference signal is relatively large, each of pulse spans of the output of the XOR is long accordingly. The long pulse spans are integrated by the LPF 102, respectively, and thus the output of the LPF 102 has a large value. Thus, an integrated value of the phase difference .delta..o slashed. between the modulated signal and the reference signal is obtained. As a result, the modulated signal which was obtained through the above-described PSK method is demodulated, that is, data reproduction is performed.
The reference voltage values of the reference top voltage and the reference bottom voltage are determined depending on a gain of the low-pass filter 102. The reference voltage values have to be determined accurately to match an output level of the low-pass filter 102. This is because demodulating accuracy of the demodulating circuit may be greatly degraded if the reference voltage values do not accurately match the output level of the low-pass filter 102.
In order to obtain such demodulating accuracy at a high level, a provision is made, in such a demodulating circuit in the related art, to appropriately adjust such reference voltage values as those of the reference top voltage and reference bottom voltage supplied by the voltage sources 120 and 121. The above provision adjusts the above reference voltage values according to various causes which affect the matching of the reference voltage values with the output level of an analog-signal producing circuit such as the low-pass filter 102. The above various causes may include offset errors and functional-value dispersion due to integrated-circuit manufacturing processes of analog-signal circuits included in the analog-signal producing circuit.